Implants. Done well, they look like real boobs. Done poorly, they look like a ball placed under the skin.
Since well done fake boobs look real, I'd say real boobs are best.
There are bad looking real boobs. There are also hideous fake boobs. In general, I'd say real are best.
Large boobs are better done fake, because it's hard to find a natural pair of large unsaggy boobs. Small boobs look best real, because they're perky and cute anyways.
Someone mentioned that real boobs don't age well. Neither do implants, which is why it's required that they be redone every 5-10 years. They sag and deflate, as well as burst/leak. This can actually cause permanent deformation.
I think if I had fake ones, I'd be afraid to sleep on my stomach for fear that they'd pop.